Backpack frame



FIG. 1 is a front elevational view of a backpack frame showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a left side elevational view thereof;

FIG. 4 is a right side elevational view thereof;

FIG. 5 is a top plan view thereof;

FIG. 6 is a bottom plan view thereof. 

The ornamental design for a backpack frame, as shown and described. 